Joining method and joining apparatus

ABSTRACT

A joining method is a method of joining a plate set composed by a plurality of stacked plate members including a first plate member and a second plate member by using a tapping screw. When the tapping screw is advanced into the plate set while the tapping screw is rotated in a state of applying a pressure onto the plate set so as to form through-holes in the first plate member and the second plate member, a first pressure applied onto the tapping screw is reduced down to a second pressure at the time when the tapping screw penetrates the first plate member.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese Patent Application No.2017-077687 filed on Apr. 10, 2017, which is incorporated herein byreference in its entirety including the specification, drawings andabstract.

BACKGROUND 1. Technical Field

The present disclosure relates to a joining method and a joiningapparatus to join a plate set composed by a plurality of stacked platemembers by using a tapping screw.

2. Description of Related Art

As a method of joining plate members by using a tapping screw, there hasbeen known a method of screwing a tapping screw into metallic members soas to form a through-hole through the metallic members and provide thisthrough-hole with a female thread, to thereby fasten the metallicmembers to each other (for example, see Japanese Patent ApplicationPublication No. 2006-177438).

SUMMARY

In the meantime, when a through-hole is formed through a plurality ofplate members by applying a pressure from one direction (from one side)with a tapping screw, in order to penetrate the first plate member, itis required to set a pressurizing force corresponding to the totalthickness of all the plate members.

In the joining method of the related art, during the process of formingthe through-hole through the plurality of plate members, a pressurizingforce (a pressurizing force corresponding to the total thickness of theall plate members) is set to be constant. Hence, when the tapping screwis driven into a portion with a smaller rigidity in the plate member,this plate member might be plastically deformed.

FIG. 8 shows an outstanding example of plastic deformation of a platemember. As shown in this FIG. 8, in a state in which a plate to bejoined 501 (hereinafter, also referred to as an upper plate 501) isstacked on a flange 502, a tapping screw 510 is driven into the upperplate 501 and the flange 502 from one direction. At this time, in thecase in which a driving position P501 of the tapping screw 510 islocated apart from a base of the flange 502, when a through-hole isformed in the flange 502 with a constant pressurizing force even afterthe tapping screw 510 penetrates the upper plate 501, plasticdeformation (such as a deformation as indicated by a two-dot chain linein. FIG. 8) might be caused to the flange 502. in such a situation, thejoining might be unacceptable, as shown in FIG. 9.

The present disclosure provides a joining method and a joining apparatuscapable of suppressing plastic deformation from being caused to theplate members, when a plate set composed by a plurality of stacked platemembers is joined by using a tapping screw.

The first aspect of the present disclosure relates to a joining methodof joining a plate set by using a tapping screw, the plate set beingcomposed by a plurality of plate members that are stacked including afirst plate member and a second plate member. The joining methodincludes: forming a first through-hole in the first plate member of theplate set and a second through-hole in the second plate member of theplate set by advancing the tapping screw into the plate set in an orderof the first plate member and the second plate member while rotating thetapping screw in a state of applying a pressure onto the plate set fromone side by the tapping screw; and forming the first through-hole andthe second through-hole with female threads, and joining the first platemember and the second plate member to each other. In such a joiningmethod, when the tapping screw penetrates the first plate member, afirst pressure applied onto the tapping screw in an axial direction ofthe tapping screw is reduced down to a second pressure that is smallerthan the first pressure.

According to the first aspect of the present disclosure, when thetapping screw penetrates the plate member, the first pressure applied tothe tapping screw is reduced down to the second pressure. By controllingthe pressurizing force in this manner, it is possible to suppressplastic deformation of the plate member more than that in the case (therelated art) in which the pressure applied to the tapping screw is setto be constant during the through-hole formation in the plate members.Through this, it is possible to join the plurality of plate members in aproper joining state.

In the first aspect of the present disclosure, the second pressure maybe a pressure corresponding to plate thicknesses of all the platemembers located more forward in an advancing direction of the tappingscrew than a front end of the tapping screw at the time when the tappingscrew penetrates the first plate member.

In the first aspect of the present disclosure, the first pressure may bea pressure corresponding to plate thicknesses of all the plate members.

In the first aspect of the present disclosure, the plurality of platemembers may be composed by the first plate member and the second platemember, and the joining method may further include reducing the secondpressure down to a third pressure that is smaller than the secondpressure when the tapping screw penetrates the second plate member.

A second aspect of the present disclosure relates to a joining apparatusthat joins a plate set by using a tapping screw, the plate set beingcomposed by a plurality of plate members that are stacked including afirst plate member and a second plate member. The joining apparatusincludes: a driver configured to hold the tapping screw; a rotarysection configured to rotate the driver around an axial center thereof;a pressurizing section configured to apply a pressure in an axialdirection of the driver onto the driver; a positional informationacquiring section configured to acquire positional information on afront end of the tapping screw held by the driver; and a controller,wherein the joining apparatus advances the tapping screw into the plateset in an order of the first plate member and the second plate memberwhile the tapping screw held by the driver is rotated in a state ofapplying a pressure onto the plate set from one side by the tappingscrew so as to form a first through-hole in the first plate member and asecond through-hole in the second plate member, form the firstthrough-hole and the second through-hole with female threads, and jointhe first plate member and the second plate member to each other.

Based on the positional information on the front end of the tappingscrew acquired by the positional information acquiring section, thecontroller is configured to execute the control to reduce a firstpressure applied to the driver by the pressurizing section down to apredetermined second pressure when the front end of the tapping screwhas moved from the position where the front end of the tapping screwcomes into contact with a surface of the first plate member of the plateset by a distance corresponding to the plate thickness of the firstplate member.

In the second aspect of the present disclosure, when the front, end ofthe tapping screw has moved from the position where the front end of thetapping screw comes into contact with the surface of the first platemember of the plate set by a distance equal to the plate thickness ofthe first plate member, it can be said that the tapping screw penetratesthe plate member. In the joining apparatus of the present disclosure,when the front end of the tapping screw has moved from the positionwhere the front end of the tapping screw comes into contact with thesurface of the first plate member of the plate set by a distance equalto the plate thickness of the first plate member, the first pressuregenerated by the pressurizing section is reduced down to thepredetermined second pressure; therefore, it is possible to suppressplastic deformation of the plate member more than that in the case (therelated art) in which the pressure applied to the tapping screw is setto be constant during the process of forming the through-hole throughthe plurality of plate members. Accordingly, it is possible to join theplurality of plate members in a proper joining state.

In the second aspect of the present disclosure, the positionalinformation acquiring section may have a distance sensor that detects amovement distance of the driver in an advancing direction of the tappingscrew. The positional information acquiring section may be configured toacquire positional information on the front end of the tapping screwbased on an output of the distance sensor.

In the second aspect of the present disclosure, the positionalinformation acquiring section may have a force sensor that detects areaction force generated when the front end of the tapping screw comesinto contact with the surface of the first plate member. The positionalinformation acquiring section may be configured to acquire thepositional information on the front end of the tapping screw based on anoutput of the force sensor

In the second aspect of the present disclosure, the second pressure maybe a pressure corresponding to plate thicknesses of all the platemembers located more forward in the advancing direction of the tappingscrew than the front end of the tapping screw when the tapping screwpenetrates the first plate member.

In the second aspect of the present disclosure, the first pressure maybe a pressure corresponding to plate thicknesses of all the platemembers.

In the second aspect of the present disclosure, the plurality of platemembers may be composed by the first plate member and the second platemember, and the controller may be configured to reduce the secondpressure down to a third pressure that is smaller than the secondpressure when the tapping screw penetrates the second plate member.

In this manner, by configuring such that the positional information onthe tapping screw is obtained based on the output of the distance sensorthat detects the movement distance of the driver, even in the case ofholding the tapping screw having a different length (length beneath thehead) to the driver, it is possible to acquire the positionalinformation on the front end of the tapping screw by using only onedistance sensor.

According to the joining method and the joining apparatus of the presentdisclosure, it is possible to suppress plastic deformation from beingcaused to the plate members when the plate set composed by the pluralityof stacked plate members is joined by using the tapping screw.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments will be described below with reference to theaccompanying drawings, in which like numerals denote like elements, andwherein:

FIG. 1 is a view showing a structure of a tapping screw;

FIG. 2 is a schematic configuration view showing one example of ajoining apparatus of the present disclosure;

FIG. 3 is a view showing a flow of a joining process;

FIG. 4A is a sectional view showing a state in the joining process:

FIG. 4B is a sectional view showing another state in the joiningprocess;

FIG. 4C is a sectional view showing another state in the joiningprocess;

FIG. 4D is a sectional view showing another state in the joiningprocess;

FIG. 4E is a sectional view showing another state in the joiningprocess;

FIG. 4F is a sectional view showing another state in the joiningprocess;

FIG. 5 is a timing chart showing one example of a control on apressurizing force;

FIG. 6 is a sectional view showing a state after the joining;

FIG. 7 is a timing chart showing another example of the control on thepressurizing force;

FIG. 8 is a view explaining a problem caused in the case of joining aplate member to be joined to a flange (plate member); and

FIG. 9 is a sectional view showing, the case in which the state afterthe joining is unacceptable.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present disclosure will be describedbased on the drawings.

[Tapping Screw]

Description will be started with a tapping screw used in the presentembodiment. As shown in FIG. 1, the tapping screw 10 is a tapping screwhaving a front end provided with no drill, and includes a head portion11, and a shaft portion 12 formed with a screw 12 a, which areintegrally formed. The head portion 11 is integrally formed with aflange 11 a, and a surface (lower surface) on the shaft portion 12 sideof this flange 11 a composes a seat surface 10 a of the tapping screw10.

[Joining Apparatus]

Next, one example of the joining apparatus to which the presentdisclosure is applied will be described with reference to FIG. 2.

The joining apparatus 100 shown in FIG. 2 includes a rotary mechanism101, a pressurizing mechanism 102, a controller 103, and an input unit104, etc.

The rotary mechanism 101 is attached to a front end of an arm 201 of arobot 200. The rotary mechanism 101 includes a motor (e.g., a servomotor) 111, and a rotary shaft 111 a of this motor 111 protrudes to theoutside (in the upward direction in FIG. 1). A support shaft 112 isprovided to a lateral part of the rotary mechanism 101. The rotarymechanism is one example of “rotary section” of the present disclosure.

The pressurizing mechanism 102 is an air cylinder, and is supported bythe support shaft 112 of the rotary mechanism 101. The pressurizingmechanism 102 includes a driver 121 located at a lower position and arotary drive shaft 122 located at an upper position in FIG. 2. Therotary drive shaft 122 is disposed in parallel to a rotary shaft 111 aof a motor 111 of the rotary mechanism 101.

The driver 121 is connected to a piston (not illustrated) inside thecylinder. The driver 121 and the rotary drive shaft 122 arespline-connected to each other (not illustrated), for example, so thatthe driver 121 is integrally rotatable with the rotary drive shaft 122.In addition, the driver 121 is movable in the axial direction (thevertical direction in the state shown in FIG. 1) relative to the rotarydrive shaft 122.

A front end of the driver 121 is provided with a socket hole (a recessedportion with a hexagonal section) 121 a into which a head portion (in ahexagon shape) 11 of the tapping screw 10 shown in FIG. 1 is fitted. Afront end portion of the driver 121 is provided with a chuck (notillustrated) to hold the tapping screw 10 fitted in the socket hole 121a. This chuck is configured to be detached from the tapping screw 10when the front end of the tapping screw 10 comes into contact with aplate member to be joined (an upper plate 1).

The rotary mechanism 101 includes a force sensor 124 to detect a forceacting in the axial direction of the driver 121, and a distance sensor125 to detect a movement distance in the axial direction of the driver121 (a movement distance in the advancing direction of the tapping screw10) Respective outputs of the force sensor 124 and the distance sensor125 are inputted into the controller 103.

A pulley (a driven pulley) 123 is attached to the rotary drive shaft 122of the pressurizing mechanism 102 in an integrally rotatable manner. Abelt 110 is wound between the pulley 123 of the rotary drive shaft 122and a pulley (a driving pulley) 113 attached to the rotary shaft 111 aof the motor 111 of the rotary mechanism 101 in an integrally rotatablemanner, so that the rotary drive shaft 122 is rotated by driving of themotor 111. Due to the rotation of this rotary drive shaft 122, thedriver 121 is rotated around its axis, driving of the motor 111 of therotary mechanism 101 is controlled by the controller 103.

In addition, the pressurizing, mechanism 102 is supplied with air froman air supply source (e.g. a not-illustrated compressor) via an aircontroller 120. By controlling the air supply to the pressurizingmechanism 102, the driver 121 moves in a driving direction, or retreatsin an inverse direction to the driving direction. This control of theair supply is carried out by an air controller 120 and the controller103. Note that the pressurizing mechanism 102 is one example of“pressurizing section” of the present disclosure.

[Controller]

The controller 103 includes a CPU (central processing unit), a ROM (readonly memory) that stores programs to control respective components, aRAM (random access memory) that temporally stores data, and aninput-output interface, and others.

The CPU is configured to execute arithmetic processing based on programsand data stored on the ROM. The ROM stores the programs and the date forcontrolling. The RAM temporally stores arithmetic results obtained bythe CPU. The motor 111 of the rotary mechanism 101, an air controller120, the force sensor 124, the distance sensor 125, the input unit 104,and the others are connected to the input-output interface.

The controller 103 controls the rotary mechanism 101 and thepressurizing mechanism 102 in the joining process of the plate members.

[Joining Process]

Next, one example of the joining process will be described withreference to FIG. 2 to FIG. 5. In this example, as shown in FIG. 4A toFIG. 4F, a tapping screw 10 is driven from one direction (one side) intoa plate set S composed by the plate member to be joined (the steelplate) 1 and a flange (a steel plate) 2 on which the plate member to bejoined 1 is stacked, to thereby join the plate member to be joined 1 andthe flange 2 to each other. In the following description, the platemember to be joined 1 is also referred to as the upper plate 1, and theflange 2 is also referred to as a lower plate 2.

(Pre-Treatment)

First, a pressurizing force is found based on a plate thickness t1 andthe material of the upper plate 1 and a plate thickness t2 and thematerial of the lower plate 2. Specifically, for example, because thejoining manner treated as the subject of the present disclosure is inthe case of using the plate member that is easily plasticallydeformable, or in a state in which the plate member is fixed on one sideso as to be easily plastically deformable (the state as shown in FIG. 8,for example), it is necessary to consider the respective platethicknesses and the respective materials of the upper plate 1 and thelower plate 2. Hence, based on the total plate thickness (t1+t2) of theupper plate 1 and the lower plate 2 and the materials thereof, apressurizing force W1 required for the tapping screw to penetrate theupper plate 1 with the upper plate 1 staked on the lower plate 2 (thepressurizing force corresponding to the total plate thickness (t1+t2))is found. In addition, based on the plate thickness t2 of the lowerplate 2 and the material thereof, a pressurizing force W2 (W2<W1)required for the tapping screw to penetrate the lower plate 2 is found.

The input unit 104 is operated so as to input, into the controller 103,the plate thickness t1 of the upper plate 1, the plate thickness t2 ofthe lower plate 2, the pressurizing force W1 corresponding to the abovetotal plate thickness, the pressurizing force W2 required forpenetration through the lower plate 2, and a pressurizing force W3 usedwhen the tapping is earned out in the through-holes (the female-threadformation). The respective plate thicknesses t1, t2, and the respectivepressurizing forces W1, W2, W3 that have been inputted are all stored onthe RAM of the controller 103.

Next, as shown in FIG. 2, the tapping screw 10 is held by the driver 121of the pressurizing mechanism 102 of the joining apparatus 100.Thereafter, the robot 200 is operated so as to place the front end ofthe tapping screw 10 onto a driving position P1 shown in FIG. 4A. Inthis state, the position of the arm 201 of the robot 200 (the positionof the rotary mechanism 101) is fixed. In this setting state, the axialcenter of the tapping screw 10 (the driving direction) is disposedvertically to the upper plate 1.

After such a pre-treatment is carried out, the following joining process(ST101 to ST105) is executed. In the following joining process, thedriving control on the motor 111 of the rotary mechanism 101 based onthe respective outputs of the force sensor 124 and the distance sensor125, the air supply control on the pressurizing mechanism 102, andothers are executed by the controller 103.

(ST101)

The air supply for the pressurizing mechanism 102 is controlled so as toadvance the driver 121 toward the upper plate 1, and detect the positionof a surface (an upper surface) 1 a of the upper plate 1. Specifically,when the front end of the tapping screw 10 comes into contact with thesurface 1 a of the upper plate 1 due to the advance of the driver 121(see FIG. 4A), a reaction force is generated, and thus the output of theforce sensor 124 is changed, Therefore, based on the position of thedriver 121 at the time when the output of the force sensor 124 ischanged (the position of the driver 121 obtained from the output of thedistance sensor 125), the position of the surface 1 a of the upper plate1 (hereinafter, also referred to as a surface position) is detected. Theposition of the driver 121 at the time when the surface position of theupper plate 1 is detected is set as a reference position.

(ST102)

From the state shown in FIG. 4A, that is, the state in which the frontend of the tapping screw 10 is in contact with the surface 1 a of theupper plate 1, the driver 121 is rotated and advanced toward the drivingdirection by the driving control on the motor 111 of the rotarymechanism 101 and the supplied-air control on the pressurizing mechanism102, as shown in FIG. 4B, while being rotated around its axial center,the tapping screw 10 is pressurized against the upper plate 1 from onedirection (T1 to T2 in FIG. 5). During this pressurizing process, at thetime (the time of T2 in FIG. 5) when a force obtained from the output ofthe force sensor 124 (the pressurizing force onto the upper plate 1)becomes the pressurizing force W1 that is previously inputted (thepressurizing force corresponding to the total plate thickness (t1+t2)),the air supply for the pressurizing mechanism 102 is controlled so as tomaintain the pressurizing force at the pressurizing force W1. In thismanner, by applying the pressurizing force W1 onto the upper plate 1while the tapping screw 10 is rotated, the upper plate 1 is pre-heated,and the front end of the tapping screw 10 is brought to advance into theupper plate 1 by the pressurizing force W1. The through-hole formationin the upper plate 1 is carried out, by this advance of the tappingscrew 10 into the upper plate 1.

(ST103)

When the front end of the tapping screw 10 penetrates the upper plate 1(when the front end of the tapping screw 10 comes into the state shownin FIG. 4C), the pressurizing force is reduced. Specifically, based onthe output of the distance sensor 125, when it is detected that thedriver 121 (the front end of the tapping screw 10) has moved by adistance equal to the plate thickness t1 of the upper plate 1, from thereference position (the position that the front end of the tapping screw10 comes into contact with the surface 1 a (one surface) of the upperplate 1) defined in the above step ST101, that is, when the front end ofthe tapping screw 10 penetrates the upper plate 1 (at the time of T3 inFIG. 5), the pressurizing force generated by the pressurizing mechanism102 is reduced down to the predetermined pressurizing force W2 that isrequired for the penetration through the lower plate 2 (the pressurizingforce corresponding to the plate thicknesses of the plate memberslocated in the advancing direction of the tapping, screw) (W1 to W2).Thereafter, the through-hole is formed in the lower plate 2 while thepressurizing force W2 is maintained.

(ST104)

The pressurizing force is reduced when the front end of the tappingscrew 10 penetrates the lower plate 2 (when the front end of the tappingscrew 10 comes into the state in FIG. 4D). Specifically, based on theoutput of the distance sensor 125, when it is detected that the driver121 (the front end of the tapping screw 10) has moved by a distanceequal to the total plate thickness (t1+t2) of the upper plate 1 and thelower plate 2, from the reference position defined in the above stepST101, that is, when the front end of the tapping screw 10 penetratesthe lower plate 2, the pressurizing force generated by the pressurizingmechanism 102 is reduced (T4 to T5 in FIG. 5). Thereafter, while thepressurizing force is set to be the pressurizing force W3 required forthe tapping, the tapping (the female-thread formation) is carried out ina through-hole 1 b of the upper plate 1 and a through-hole 2 b of thelower plate 2, as shown in FIG. 4E (T5 to T6 in FIG. 5).

(ST105)

After the tapping is carried out in the through-hole 1 b of the upperplate 1 and the through-hole 2 b of the lower plate 2, at the time whenthe seat surface 10 a of the tapping screw 10 is seated on the surface(the upper surface) of the upper plate 1 (FIG. 4F), the pressurizingforce is released and a fastening torque is applied onto the tappingscrew 10, thereby to fasten the upper plate 1 and the lower plate 2 toeach other (T6 to T7 in FIG. 5),

The steps ST101 to ST105 in FIG. 3 are executed by the controller 103,to thereby embody “position information acquiring section” and“controller” of the present disclosure.

<Effects>

As aforementioned, according to the present embodiment, when the tappingscrew 10 is advanced while the tapping screw 10 is rotated in the stateof applying the pressure force onto the plate set S from one directionso as to form the through-holes 1 b, 2 b in the upper plate 1 and thelower plate 2 of the plate set 5, at the time when the front end of thetapping screw 10 penetrates the upper plate 1, the pressurizing forcegenerated by the pressurizing mechanism 102 is reduced down to thepressurizing force W2 corresponding to the plate thickness t2 of thelower plate 2 (W1 to W2). By controlling the pressurizing force in thismanner, it is possible to suppress plastic deformation of the lowerplate 2 more than that in the case of controlling the pressure appliedonto the tapping screw to be constant (the related art), during thethrough-hole formation in the plate members. With this configuration, asshown in FIG. 3, even if the driving position P1 of the tapping screw 10is located apart from a base of the flange, it is possible to suppressthe plastic deformation of the lower plate 2, and thus, as shown in FIG.7 it is possible to join the upper plate 1 and the lower plate 2 to eachother in a proper joining state.

Other Embodiments

Note that the embodiments disclosed herein are intended to beillustrative in all respects and should not be construed as the basisfor restrictive interpretations. Therefore, the technical scope of thepresent disclosure is not intended to be interpreted based on only theabove-described embodiments, but rather is defined based on thedescription in the claims. Moreover, all changes within meanings andscopes equivalent to the claims are embraced by the technical scope ofthe present disclosure.

In the above embodiments, the case of joining the plate to be joined tothe flange (the case of driving the tapping screw into the portion ofthe plate member having a smaller rigidity) has been described, but thepresent disclosure is not limited to this, and the present disclosure isalso applicable to any other manner as far as the joining is for theplate set composed by a plurality of stacked plate members.

In the above embodiments, the example of applying the present disclosureto the case in which the two plate members (the upper plate 1 and thelower plate 2) are joined to each other has been described, but thepresent disclosure is not limited to this, and is, also applicable tothe case in which three or more plate members are joined,

For example, in the case of joining a plate set composed by threestacked plate members of an upper plate, an middle plate, and a lowerplate, as shown in FIG. 7, the pressurizing force for forming thethrough-hole in the first plate member (the upper plate) located in thedriving direction of the tapping screw is set to be a pressurizing forceW11 corresponding to the total plate thickness of the three platemembers. Next, when the tapping screw penetrates the upper plate, thepressurizing force is reduced down to a pressurizing force W12corresponding to the plate thicknesses (the total plate thickness) ofthe two plate members (the middle, the lower plate) located in theadvancing direction of the tapping screw (W11 to W12). When the tappingscrew penetrates the middle plate, the pressurizing force is reduceddown to a pressurizing force W13 corresponding to the plate thickness ofone plate member (the lower plate) located in the advancing direction ofthe tapping screw (W12 to W13).

In this manner, every time the tapping screw penetrates each platemember, the pressurizing force is reduced, to thereby reduce thepressurizing force stepwise during the advance of the tapping screw,depending on the plate thicknesses of the plate members located in theadvancing direction of the tapping screw. Through this, when the plateset composed by the three plate members is joined, it is possible tomore collectively suppress the plastic deformation of the plate members.

In addition, in the joining apparatus 100 shown in FIG. 2, for example,in the case of joining the plate set of the three stacked plate membersincluding the upper plate, the middle plate, and the lower plate, whenthe front end of the tapping screw 10 has moved due to the advance ofthe driver 121, from a position where the front end of the tapping screw10 comes into contact with one surface of the upper plate by a distanceequal to the plate thickness (when the front end of the tapping screw 10penetrates the upper plate), the pressurizing force is reduced down to apressurizing force corresponding to plate thicknesses of the two plates,that is, the middle plate and the lower plate. Next, due to the advanceof the driver 121, when the front end of the tapping screw 10 has movedfrom a position where the front end of the tapping screw 10 comes intocontact with one surface of the middle plate by a distance equal to theplate thickness (when the front end of the tapping screw 10 penetratesthe middle plate), there may be executed a control to reduce thepressurizing force down to a pressurizing force required for thepenetration through the lower plate. The present disclosure isapplicable to the case of joining four or more plate members.

In the aforementioned embodiments, with respect to the detection of theposition of the surface of the upper plate 1, the position of thesurface of the upper plate 1 is detected by detecting a reaction forcegenerated when the front end of the tapping screw 10 comes into contactwith the surface of the upper plate 1 by using the force sensor 124; butthe present disclosure is not limited to this. For example, the positionof the surface of the upper plate 1 may be detected by using anon-contact-type sensor with a laser light, an infrared light, etc., orby using a contact-type sensor with a cantilever, or the like.

In the aforementioned embodiments, the distance sensor to detect themovement distance of the driver is provided so as to obtain thepositional information on the front end of the tapping screw based onthe output from the distance sensor, but the present disclosure is notlimited to this. There may be provided a sensor to detect the positionof the front end of the tapping screw so as to obtain the positionalinformation on the front end of the tapping screw based on the outputfrom this sensor.

In the aforementioned embodiments, the air cylinder is used as thepressurizing mechanism of the joining apparatus; but the presentdisclosure is not limited to this, and as the pressurizing mechanism,there may be used a mechanism in combination of a motor (such as a servomotor) and a rotation-to-translation conversion mechanism (such as aball screw, or another actuator such as a hydraulic cylinder.

In the aforementioned embodiments, the example of using the tappingscrew 10 whose head portion 11 has a hexagonal shape is described, butthe manner of the head portion of the tapping screw is, not limited to aspecific one, and for example, and may be applicable to the case ofjoining the plate members using a tapping screw having a head portionwith a cross recess.

In the aforementioned embodiments, the case of applying the presentdisclosure to the joining of the steel plates is described, but thepresent disclosure is not limited to this, and may be applicable tojoining of metallic plate members other than steel plates, or platemembers made of resin.

The present disclosure is effectively usable in the joining method ofjoining the plate set composed by the plurality of stacked plate membersby using the tapping screw.

What is claimed is:
 1. A joining method of joining a plate set by usinga tapping screw, the plate set being composed by a plurality of platemembers that are stacked including a first plate member and a secondplate member, the joining method comprising: forming a first throughhole in the first plate member of the plate set and a secondthrough-hole in the second plate member of the plate set by advancingthe tapping screw into the plate set in an order of the first platemember and the second plate member while rotating the tapping screw in astate of applying a pressure onto the plate set from one side by thetapping screw; and forming the first through-hole and the secondthrough-hole with fez ale threads, and joining the first plate memberand the second plate member to each other, wherein when the tappingscrew penetrates the first plate member, a first pressure applied ontothe tapping screw in an axial direction of the tapping screw is reduceddown to a second pressure that is smaller than the first pressure. 2.The joining method according to claim 1, wherein the second pressure isa pressure corresponding to plate thicknesses of all the plate memberslocated more forward in an advancing direction of the tapping screw thana front end of the tapping screw at a time when the tapping screwpenetrates the first plate member,
 3. The joining method according toclaim 1, wherein the first pressure is a pressure corresponding to platethicknesses of all the plate members.
 4. The joining method according toclaim 1, wherein the plurality of plate members are composed by thefirst plat; member and the second plate member, and the joining methodfurther comprises reducing the second pressure down to a third pressurethat is smaller than the second pressure when the tapping screwpenetrates the second plate member.
 5. A joining apparatus that joins aplate set by using a tapping screw, the plate set being composed by aplurality of plate members that are stacked including a first platemember and a second plate member, the joining apparatus comprising: adriver configured to hold the tapping screw; a rotary section configuredto rotate the driver around an axial center of the driver; apressurizing section configured to apply a pressure in an axialdirection of the driver onto the driver; a positional informationacquiring section configured to acquire positional information on afront end of the tapping screw held by the driver; and a controllerconfigured to execute a control to reduce a first pressure applied ontothe driver by the pressurizing section down to a predetermined secondpressure based on the positional information on the front end of thetapping screw acquired by the positional information acquiring section,when the front end of the tapping screw has moved from a position wherethe front end of the tapping screw comes into contact with a surface ofthe first plate member of the plate set by a distance corresponding to aplate thickness of the first plate member, wherein the joining,apparatus advances the tapping screw into the plate set in an order ofthe first plate member and the second plate member while the tappingscrew held by the driver is rotated in a state of applying a pressureonto the plate set from one side by the tapping screw so as to faun afirst through-hole in the first plate member and a second through-holein the second plate member, form the first through-hole and the secondthrough-hole with female threads, and join the first plate member andthe second plate member to each other.
 6. The joining apparatusaccording to claim 5, wherein the positional information acquiringsection having a distance sensor that detects a movement distance of thedriver in an advancing direction of the tapping screw, the positionalinformation acquiring section being configured to acquire positionalinformation on the front end of the tapping screw based on an output ofthe distance sensor.
 7. The joining apparatus according to claim 5,wherein the positional information acquiring section having a forcesensor that detects a reaction force generated when the front end of thetapping screw comes into contact with the surface of the first platemember, the positional information acquiring section being configured toacquire the positional information on the front end of the tapping screwbased on an output of the force sensor.
 8. The joining apparatusaccording to claim 5, wherein the second pressure is a pressurecorresponding to plate thicknesses of all the plate members located moreforward in an advancing direction of the tapping screw than the frontend of the tapping screw when the tapping screw penetrates the firstplate member.
 9. The joining apparatus according to claim 5, wherein thefirst pressure is a pressure corresponding to plate thicknesses of allthe plate members.
 10. The joining apparatus according to claim 5,wherein the plurality of plate members are composed by the first platemember and the second plate member, and the controller is configured toreduce the second pressure down to a third pressure that is smaller thanthe second pressure when the tapping screw penetrates the second platemember.